Fermentation of concentrated skim‐milk. Effects of different protein/lactose ratios obtained by ultrafiltration–diafiltration

Defatted milk was ultrafiltered–diafiltered in a 20 kDa polysulphone flat membranes UF unit to obtain concentrated milk with different protein/lactose ratios. Different samples of concentrated–diafiltrated milks with protein concentration between 4·7% and 8·9% and lactose concentration between 2·7 and 0·8% (protein/lactose ratios between 1·7 and 9·5) were fermented (Lactobacillus bulgaricus and Streptococcus thermophilus) to obtain yoghurt-like products. Firmness of the coagulum as well as pH, colony count and organoleptic test were considered in the final products. Concentrated-diafiltrated milk with lactose and protein contents lower than 2% and 6·5%, respectively, lead to a very liquid product. Enriched protein low-lactose fermented milks can be obtained by this technology.